The present invention relates to weighing machines, and in particular to storage hoppers which receive and discharge product to be weighed in the course of weighing operations. Such hoppers, for example, may be a weighing hopper which is coupled to a transducer to generate a weight signal for subsequent processing. The storage hopper is especially useful in combination weighing machines in which a plurality of the hoppers receive and discharge product in a cyclic operation to generate charges of product for packaging.
Weighing machines which are commonly used in packaging operations generally include a plurality of weighing hoppers which periodically receive product to be weighed and discharged after a weight measurement has been taken. U.S. Pat. No. 2,387,585 illustrates a conventional packaging machine in which the weighing hoppers are an integral part of the charge-forming operation. More recently combination weighers of the type shown in U.S. Pat. No. 4,466,500 have assumed the charge-forming and weighing functions because of the high speed at which the machines can operate without reducing the accuracy of the weighing function. In combination weighers a plurality of weighing hoppers are used to continuously weigh small quantities of product, and the quantities are then combined in selected combinations to form charges of product closely approximating a given target weight.
In all types of weighing machines which generate signals indicative of the measured weight, it is preferable that such signals represent as much of the weight of product as possible and as little of the weight of the hopper as possible. For this reason range springs such as shown in U.S. Pat. No. 4,550,792 are sometimes added to the support mechanism for the hopper to carry a substantial portion of the "dead" weight of the hopper. Thus, the weight of the hopper does not appear in the signal derived from the strain gauge or other weight sensor and the total weight capacity of the sensor is confined to the range of product weights anticipated. Unfortunately, however, range springs are not permitted by many government or administrative regulations and, without other counterbalances for the dead weight of the hopper, as little as 20% of the weight signal may represent the useful load of product. Since the errors due to hysteresis and linearity are dependent on the total weight signal from the sensor, significant error can remain in the product weight component after the dead weight component has been subtracted.
In many weighing machines the hoppers frequently have a construction with the support structure, door hinges and operating linkages secured or suspended from the body. Examples of such hoppers are shown in U.S. Pat. Nos. 4,398,612 and 4,635,831 and 4,874,048. With a construction and hopper assembly of this type, the body itself must have a heavy gauge metal sufficient to carry the bearing loads and stresses that arise from the associated loads carried by the other structure and mechanisms. Such hopper structures are, therefore, relatively heavy structures and contribute to a significant portion of the weight measurements. The same hopper structures inhibit the use of different and less expensive materials, and are costly to manufacture due to the materials used and the polishing and other work that is needed for a sanitary finish.
In other designs, the hoppers have a partial frame for supporting the hopper body in part. Such hoppers are shown in U.S. Pat. Nos. 4,499,962, 4,527,647 and, 4,545,446. The partial frames reinforce the hopper body and thereby allow some reduction in weight through lower gauge metals, but still allocate some of the load bearing functions to the body. With such designs, the bodies must have sufficient strength to carry loads other than those from the weight of product, and the bodies and reinforcing frames are integrated structures inseparable from one another.
A further requirement of many weighing machines having a plurality of hoppers is ease of inspection, maintenance and disassembly for cleaning. For example, customers often specify that the hoppers must be removable from the weighing machine without tools. In accordance with these design requirements, weighing machines have releasable couplings between the hoppers and the machine frames. Examples of such hoppers and their couplings are shown in U.S. Pat. Nos. 4,398,612, 4,499,962 and 4,527,647. Because of the environment and the constant state of vibration that exists when such hoppers are operating, it is essential that the couplings hold the scales rigidly and securely to the machine. The objectives of a secure and rigid fastening along with ease of disassembly are generally in conflict and accordingly designing such couplings is a formidable task.
To minimize the "dead" weight of a hopper and at the same time to permit the hopper to be readily disassembled from the machine, it is customary to mount the actuating mechanism for opening and closing a discharge door on the frame of the machine and to transmit the driving motions to the door through an interrupted linkage. In this fashion the weight of the actuator does not become part of the sprung weight of the hopper. Examples of interrupted linkages are shown in U.S. Pat. Nos. 4,499,962, 4,527,647, 4,635,831 and 4,874,048. In such linkages the actuator is a uni-directional actuator which simply pushes one end of the operating rod against an operating linkage to cause the door to open, and a spring mechanism connected with the operating linkage returns the door to the closed position. Consequently, the spring is relied upon to hold the door closed and locked in opposition to the weight of product and the impact loads arising when product falls into the hopper.
It is accordingly a general object of the present invention to provide a weigh hopper which has a construction which allows different and lighter materials to be used in the non-stressed portions of the hopper so that the hopper has less weight and can be constructed for removability and at substantial cost saving over the conventional construction methods.
It is a further object of the present invention to provide a quick-release coupling which maintains the hopper rigid and securely connected to the machine frame during weighing operations and which releases the hopper from the machine frame simply and without tools when necessary for cleaning, repair or inspection.
It is still a further object of the present invention to provide a hopper with a discharge door and operating linkage that is easily released during hopper removal and, at the same time, securely closes and locks the door during cyclic filling and discharging operations.